This invention relates to a roof bit assembly. More particularly, this invention relates to a roof bit insert assembly for securely connecting an insert onto a bit body.
Roof bits are used in a violent environment in which heavy loads and torques are continually applied to the roof bits at a high rate. One inch diameter roof bits are commonly operated at about 250-600 rpm and 1000-8000 pounds thrust. These conditions also generate excessive heat at the tip end of the roof bit assembly.
A hard wear resistant insert for use in a center vacuum rotary drill bit is typically formed of cemented tungsten carbide and is secured by brazing the insert within a slot formed within a top working surface of the body of the rotary drill bit. The hard wear resistant insert secured within the top working surface of the rotary drill bit body typically wears or dulls before the drill bit body becomes worn and unusable such that the user of the center vacuum rotary drill bit must either replace the entire rotary drill bit, attempt to replace the insert with a new insert of similar size, or attempt to resharpen the insert. Replacement of the entire center vacuum rotary drill bit is costly and resharpening of the insert may only be performed a limited number of times before the top working surface of the drill bit body frictionally engages the material to be cut.
In U.S. Pat. Nos. 5,310,015; 5,184,925 and 5,226,489, to Woods et al., a roof bit assembly is disclosed wherein a support bar is positioned between the insert bit and bit body for adjusting the height that the insert bit extends beyond the top surface of the roof bit body. In U.S. Pat. No. 5,184,925, the insert body has a semicircular notch 50 therein and the support bar has a corresponding convex portion for cooperating with the depression. The support bar in these roof bits results in additional assembly time during original manufacturing or when replacing inserts onto a bit body. The assembly of these roof bits can be even more time consuming and cumbersome in the field. The support bar also requires maintaining additional parts and undesirable expanding inventories.
In U.S. Pat. No. 4,356,873, to Dziak, an insert is provided with a notch that cooperates with a projection 38 on a retainer clip that must be fixed into a slot of the bit body. The retainer clip is laterally held in position in the bit body slot by flanges 40 that cooperate with the exterior cylindrical wall of the bit body.
There is a need in the industry for a mining roof bit that requires a minimum number of parts, is easy to assemble accurately, braze, and comparatively inexpensive in cost. There is also a need in the industry for an insert that is securely held in the bit body during drilling operations and is not as readily knocked loose from the bit body.
Briefly, according to this invention there is provided a center vacuum rotary drill bit including a rotary drill bit body and an insert. The rotary drill bit body includes a top working surface having a slot extending transversely across the top working surface. The slot includes a first sidewall, an opposite second sidewall and a bottom surface extending between the first sidewall and the second sidewall. The insert includes a pair of oppositely facing generally parallel side surfaces, a pair of oppositely facing end surfaces extending between and interconnecting the side surfaces, a pair of oppositely inclined top surfaces extending between and interconnecting the side and end surfaces, and a bottom surface extending transversely to and interconnecting the side and end surfaces. The bottom surface of the slot has a projecting key that cooperates with a depression keyway of the insert.
In one embodiment, generally vertical sidewall surfaces of a depression keyway and vertical sidewall surfaces of a projection key provide mechanical resistance against lateral movement of the cutting insert. This mechanical resistance reduces displacement of the insert with respect to the bit body during brazing and thus a high quality, accurately aligned drill bit assembly is provided by applicants"" invention. Additionally, it is believed that during drilling operations the vertical surfaces assist in resisting lateral displacement caused by lateral force loads and torques applied to the cutting ends of the insert.
The depression keyway is designed so as to minimize the effect of the axial load on the insert by avoiding sharp corners which tend to amplify nominal stress and provides for an insert of a proper carbide density after pressing.
Integral bit body protuberance means contact the insert along the slot, the protuberance means are cylindrical ridges, semispherical, circular, prisms, angular or an equivalent shape or form. The protuberance means adequately separate the insert within the slot and permit braze flow between the insert and slot. The protuberance means enhance braze flow when braze is fed between the insert body and bit body.
The overall bottom surface area of the cemented tungsten carbide insert is significantly increased by the keyway depression. The key and keyway increase the surface contact area between the bottom surface area of the insert and cooperating bottom surface of the slot. The increased surface contact area allows for additional braze to be applied between the insert and bit body slot enhancing the strength of the bond between the insert and bit body.
Another added benefit of the depression keyway is that the depression keyway in the tungsten carbide reduces the amount of tungsten carbide or other material needed to manufacture the insert. In comparison to an insert bit dimensionally equivalent in shape and size to applicants"" insert without a keyway, 30%-50% by weight less tungsten is needed to make applicant""s insert bit. Cemented tungsten carbide is relatively expensive and applicants, insert design significantly reduces material costs.
The invention requires a minimum number parts for assembly, reduces material cost and provides for a strong and precisely assembled roof bit.